Lithographic processes are well known in the art for use in the manufacture of semiconductor devices. Lithography generally involves transferring a desired pattern, such as a circuit pattern, through a resist layer onto an underlying silicon substrate. The first step of the process generally involves forming a resist layer on the substrate layer. The resist layer is then exposed to patterned radiation to cause dissolution differentiation in the resist layer. The resist layer is then developed generally with liquid developer to form a positive tone or negative tone pattern in the resist layer. Negative tone images are very important in certain lithographic patterns, such as for microprocessors. The pattern is then transferred to the underlying silicon substrate with transfer techniques such as etching or ion implantation. However, there is in the industry environmental concerns with liquid developer waste streams. Processes for the dry development of the resist layer have been discovered. However, due to technical considerations, these processes have not been widely adopted in the industry.
There is also a desire in the industry for higher circuit density in microelectronics devices which are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. There is a goal in the industry to reduce the feature size to below 0.20 microns. The use of shorter wavelength radiation (e.g., deep UV; e.g., 193) rather than the currently employed md-UV spectral range (e.g., 300-450 nm) offers the potential for this higher resolution. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, almost all art-known photoresists absorb strongly in the deep UV, making them unsuitable as resist materials. This problem has been, in some cases, avoided by the use of electron beam lithography. Electron beam lithography enables the use of resist materials which are opaque in the UV spectrum. However, e-beam lithography has associated problems of scattering with associated image distortion, which have not yet been resolved.
Nishikawa, U.S. Pat. No. 4,944,837, discloses lithographic imaging of polymethyl methacrylate (PMMA) resist with an electron beam and development of a positive tone image by removal of resist material degraded by e-beam exposure with supercritical fluid. Although Nishikawa has addressed environmental concerns associated with resist liquid development, resolution and image distortion are still of concern.
It is therefore an object of the present invention to provide an improved lithographic process for negative tone images using deep UV light which provides improved resolution and images without environmental issues.
Other objects and advantages will become apparent from the following disclosure.